What types are uncommon in the current metagame? Why?
Think of a type with no obviously outstanding traits and legions of mediocre members. Imagine there was a type so overlooked and underused, no CAP had ever been designated it before and only one OU Pokemon belongs to it, despite it being the second most common type in the game. Normal not only practically defines "underused", it is the purest slate possible for us to create this concept, perhaps especially if Normal is not the type we are looking to bolster.
What positive features do these underused types possess?
Normal operates almost entirely outside the tangled web of checks and counters created by the type chart, and many normal types have good move pools. If we wanted to make a Pokemon versatile enough to counter an array of dominant types and support some lackluster ones without belonging to any of them, Normal would surely be the way to go.
What existing Pokémon individually affect the usage of entire types?
There are so many ways to approach this question, I'm not sure which to pick. Many pokemon with remarkable strengths sport 4x weaknesses that grant special niches to pokemon like Mamoswine and necessitate coverage moves on others. Magnezone has just the right tools to take care of all the bulky steel types in OU. Any one of the powerful dragons could be said to be the one that forces every team to have a steel type. Politoed brings a dizzying myriad of exploitable buffs to certain types, and to a lesser extent so do the other weather inducers. The list goes on and on.
What ways are there to change a typing’s usage outside of simply countering/being countered by them?
Weather support is a prime example. Offensive and defensive type synergy are both very helpful. Taxing the opposing pokemon's health upon entry just for being the wrong type is obviously effective at reducing those types' usage. Even just introducing a new type combination can change the dynamics between types.
I'm sure I'll have more to add later, but for now I'll leave it here.